This invention relates to a fifth wheel coupling for mounting on a towing vehicle for co-operation with a waisted king-pin mounted on a trailer.
A known form of fifth wheel coupling is shown in GB 1377741 wherein a pivotally mounted combined hook and sensor arm and a locking member are both mounted on the same side of a slot in a plate arranged to receive a king-pin of a trailer.
The king-pin has an enlarged portion and a waisted portion and when correctly aligned the king-pin engages the sensor arm as the king-pin is advanced into the slot in the plate and the hook moves from an open position to a closed position in which the hook engages the waisted portion of the king-pin.
The aforesaid coupling also includes a pivotally mounted locking member which is moveable from a withdrawn position in which the combined hook and sensor arm are held in the open position to a closed position which retains the hook in the closed position. In moving from the withdrawn position to the closed position the locking member rotates and is axially slidable.
Another known form of fifth wheel coupling is disclosed in GB2236729 wherein a pivotally mounted hook and an axially slidable locking member are mounted on opposite sides of the slot in the mounting plate which is designed for reception of the king-pin of the trailer. The limbs of the hook engage around the waisted portion of the king-pin as it enters the slot. One limb of the hook is formed with a recess through which the enlarged lower portion of the king pin will pass if the king pin is incorrectly aligned and too high for accurate engagement by the hook.
The present invention proposes an improved arrangement of fifth wheel coupling for mounting on a towing vehicle for co-operation with a waisted king-pin mounted on a trailer, the coupling including a pivotally mounted member comprising a hook and a sensor arm and an axially moveable locking member, the pivotally mounted member and the locking member being arranged on opposite sides of a slot in a plate on which the pivotally mounted member is mounted and the hook being moveable between an open position and a closed position in which the hook can co-operate with the waist of a king-pin to retain the king-pin in the slot, characterised in that the hook and the sensor arm are rigid one with another and are offset from each other so as to extend in different planes so that the sensor arm, when the hook is in the open position, is arranged to be engaged by the enlarged portion of a correctly aligned king-pin entering the slot, whereupon the hook is moved towards its closed position.
The hook can be biased from its closed position to its open position, the open position being such that a king-pin can move freely out of the slot.
The locking member can be moveable between a withdrawn position clear of the slot and a locking position in which the hook is maintained in its closed position.
The free end of the sensor arm can be in contact with a cam surface on the locking member when the hook is in the open position.
The locking member can be mounted on a biased linkage mechanism, the linkage mechanism including a first detent which is engaged when the sensor arm is released from the locking member thereby preventing movement of the locking member.
The linkage mechanism can also include a second detent which can be manually engaged to draw the locking member across the slot thereby allowing the king-pin to move relative to the slot and rotate the pivotally mounted member.
The king-pin can be arranged to contact the locking member as it moves relative to the slot thereby causing the locking mechanism to become disengaged from the second detent.
The locking member can be biased towards its locking position.
The locking member can be moveable from left to right from its withdrawn position to its closed position and the pivotally mounted member is rotatable in a clockwise direction from its open position to its closed position.
The invention can further comprise a towing vehicle fitted with a fifth wheel coupler as defined above and a trailer having a waisted king-pin, the trailer being coupled to the towing vehicle by co-operation of the hook of the coupling with the waist of the king-pin.